An anti-lock hydraulic brake system for an automotive vehicle with a master brake cylinder to which wheel brake cylinders are connected by way of one or more brake lines. A pedal-operated power booster is fed by a pressure fluid source and is inserted upstream of the master brake cylinder. The system includes a booster piston one end face of which confines a resetting chamber and the other end face thereof confines a booster chamber. The resetting chamber communicates with the booster chamber by way of a normally closed change-over valve which is closed in the de-energized state. The chamber further communicates with an unpressurized compensating reservoir by way of one or more parallel connected, normally open change-over valves which are opened in the de-energized state.
A brake system of this type is known which, for slip control, controls the braking pressure by way of the time-multiplex method, (German published patent application No. 33 17 629). For this purpose, electromagnetically actuatable multidirectional control valves are inserted into the pressure-fluid conduits connecting the wheel brakes individually and/or in pairs to the braking pressure generator, the valves open or close the pressure-fluid passage. Further, control valves produce an auxiliary force which counteracts the pedal force and which for slip control reduces the force applied on the pistons in the master brake cylinder and thus the braking pressure generated in the master brake cylinder. when all wheels run stably, all wheel valves. inserted between the braking pressure generator and the wheel brakes are switched to open passage and thus communicate with the master brake cylinder. In the event of an imminent locked condition at any one of the wheels, the wheel valves leading to the other wheels will be changed over to close for a short time so that only the pressure-fluid circuit of the imminently locking wheel remains connected to the master brake cylinder. An auxiliary force is built up which compensates the (auxiliary-force-assisted) brake-pedal pressure partially or completely, whereby the braking pressure is reduced at the imminently locking wheel which is the sole one connected. The pressure remains constant in this phase in the other wheels. After the pressure has been reduced and the pressure level desired is attained, the reduced pressure at that wheel which became unstable at first will be maintained constant by a valve change-over. After the decrease of the counter force and the renewed pressure development in the master brake cylinder the braking pressure build-up is continued in the other wheel brakes. This way the braking pressure at each wheel successively can be set to the desired value calculated by the associated electronics.
To generate an auxiliary force counteracting the pedal force, the booster piston confines a resetting chamber which is diminished upon brake application. In the absence of brake slip control, this resetting chamber communicates with the unpressurized compensating reservoir by way of open change-over valves. Thus, the volume contained in the resetting chamber can be supplied into the compensating reservoir without any effort. In case it becomes necessary to reset the booster piston because of a brake slip control action, the resetting chamber is furnished with pressurized pressure fluid by switching the normally closed change-over valve. To achieve this, the resetting chamber can be in direct communication with the pressure fluid source and, on the other hand, with the booster chamber of the brake power booster. Alternatively, the pressure of the pressure fluid is conformed to the pedal force so that the pedal resetting will not occur abruptly as in the first instance. The present invention is directed to this alternative case.
This arrangement is disadvantageous. Namely, if the change-over valves which establish the connection to the compensating reservoir are not able to re-assume their open position after a resetting action, for example, because one valve part is jammed, then the resetting chamber is locked hydraulically. As a result, the booster piston is not able to move and no braking effect can be exerted any longer. To avoid this it has been proposed to connect several change-over valves in parallel so that at least one change-over valve will re-open and that thus the connection to the compensating reservoir is maintained. This means that the system is redundant. It may occur, however, that one or more of the valves becomes jammed so that the system is no longer redundant. This defect cannot be recognized easily, since the still intact valve provides full readiness for use. The previously known means do not permit detection of whether one of the valves has failed and thus a redundant system is not provided. If subsequently the second valve also fails, this will occur without warning, and the driver will be confronted with an unbrakable vehicle. It is therefore an object of the present invention to create a system which checks the switch condition and the operability of the valves. This affords the possibility of issuing a corresponding alarm signal to the driver upon failure of a valve.